Osmolality is a measurement of the concentration of solute particles in a solvent. Osmometry measurements by the method of freezing point depression are conducted by supercooling a quantity of sample solution in a container, freezing the solution, and measuring its equilibrium melting temperature via a temperature probe that is inserted into the sample solution. The equilibrium melting temperature is representative of the concentration. The integrity of the sample concentration, cleanliness of the probe and, position of the probe within the sample volume are critical to obtaining accurate measurements.
There are many different size and style sample containers available for these measurements. Most containers are small (up to 1 mL), open-top plastic cups. Closed sample cups are available, but for these, the top must be opened or removed prior to running the test. For the occasional measurement of osmometry samples, these formats work quite well. However, many laboratories process several samples at about the same time and different instruments exist on the market that allow for the staging of multiple samples. For these workflow formats, the open and conventional closed-top sample cups are problematic.
The open cup allows sample solvent to evaporate from cups that have been filled but are waiting to be tested. Evaporation of the solvent concentrates the sample solution, yielding an artificially high test result. The open cup also has the potential for allowing dust and other airborne particles to contaminate the sample. Further, open sample cups may pose a biological hazard in the event that they are spilled.
The closed-top cups address the evaporation and contamination issues, but they introduce another complication in that they require operator intervention to open the cup immediately before testing. This intervention largely eliminates the possibility of staging multiple samples in advance.
Another important aspect to osmometry measurements is cleaning of the probe between samples. If the probe is not adequately cleaned, sample residue from one test can contaminate subsequent samples (commonly known as “carryover”). The predominant cleaning method currently used is to manually wipe the sensor probe with an absorbent wipe or pad of some sort. This separate step adds complication to the instrument and for the user, and increases overall test time. It is also imperfect in that the operator can forget to do the cleaning entirely. Also, operators often use slightly different techniques to clean, resulting in operator to operator variability. Automated systems often rely on a separate cleaning fluid flush or similar approach, again adding complexity and time to the overall process and instrument.